That's awfully high: You should be seen well before that, child or adult. [1] I have only had such a fever once in my adult life - and I had serious cognitive defects. I couldn't drive. I fell asleep while while waiting for the doctor, who proceeded to have me take 800mg of children's liquid ibuprofen and kept me there to make sure my fever started to go down, otherwise they were going to send me to the ER. My fever was 103.1 (39.5C). I'd think at 42C, I'd be in serious, serious trouble.

All that said, a low-grade fever isn't all that harmful. I still tend to take something. I used to try to tough it out before the experience above, but also found that without the body aches and headache that comes with the fever, I slept better while sick, even though I still would sleep for hours. I also tended to feel better a little sooner. Of course, this isn't data, and I've read the stuff about the beneficial fever before, but until general medical advice tells me to do differently, I'll stick to comforts.

[1] https://www.mayoclinic.org/diseases-conditions/fever/symptom...